Global Biogeochemical Cycles最新文献

Dissolved manganese (dMn) is an essential bioactive element required for marine organisms. Redox condition determines its solubility and its solid phase removal from seawater. It displays a typical scavenging type profile in the Indian Ocean with an elevated concentration in the Oxygen Minimum Zone (OMZ) of the Bay of Bengal (BoB). The surface dMn decreases southward in the BoB, and its concentration gradient correlates well with salinity because of the enormous riverine influx. Reductive dissolution of Iron-manganese (Fe-Mn) oxyhydroxides-rich sediments brought by the Ganga-Brahmaputra rivers enriches dMn in the bottom waters of the shelf regions (∼25 nM), which gets advected to the open ocean through cross-shelf transport. The atmospheric input is the prominent source of dMn in the BoB. Transport of the Indonesian Through Flow waters supplies high dMn in the surface waters of the Central Indian Ocean Basin. Internal cycling seems to control the dMn distribution in the water column in addition to its external sources. Water column denitrification increases dMn in the OMZ waters of the BoB through the reductive dissolution of sinking Mn oxide particles under the prevailing suboxic conditions. The presence of two sub-surface peaks of dMn associated with nitrite maxima suggests active denitrification in the OMZ waters of the BoB, similar to the Arabian Sea. The interaction of circulating fluid with subducting Fe-Mn-rich crusts enriches the deep water dMn in the Java Sumatra region. Further, the hydrothermal activity over the Southeast and Central Indian Ridges contributes significantly to the dMn budget of the deeper waters.

Marine oxygen deficient zones (ODZs) play a major role in the Earth's biogeochemical cycles and are responsible for nitrogen and sulfur removal from the oceans. Microbial-reducing reaction processes generate nitrite (NO₂⁻) and sulfur compounds as intermediaries that may accumulate in these zones. Current assessments on microbial transformations inside ODZs are based on shipboard measurements, and there are no well-resolved seasonal to annual observations or high-resolution vertical sampling that would characterize variability. Here, we propose an alternative statistical approach to analyze the raw output of the nitrate sensor from BGC-Argo floats with the ability to detect NO₂⁻ and thiosulfate (S₂O₃²⁻) concentrations in addition to nitrate. The new approach provides data with great vertical and spatiotemporal resolution. The method can be applied to UV-spectrometer output data from SUNAs and ISUS nitrate sensors commonly deployed on various observing platforms. We validated the technique in the field by matching shipboard NO₂⁻ bottle data with float data from the Eastern Tropical North Pacific (ETNP) and Eastern Tropical South Pacific (ETSP) ODZs. We then show a complete time series of three floats as study cases. The ability to detect NO₂⁻ and S₂O₃²⁻ concomitantly with other key chemical variables (i.e., oxygen, pH, and bio-optics) at such fine scale allows for novel insights into the nitrogen and sulfur cycling of ODZs and processes driving these cycles. This new approach will enable fine-scale remote quantification of NO₂⁻ and S₂O₃²⁻ to support a better understanding of the biogeochemical transformations happening inside these already-expanding deoxygenated regions.

Marine Carbon Dioxide Removal (mCDR) will likely play a role in efforts to keep global warming below 2°C. mCDR methods create a deficit in dissolved seawater CO₂ relative to the unperturbed counterfactual. This seawater CO₂ deficit induces either an uptake of atmospheric CO₂ or reduced CO₂ outgassing into the atmosphere. The immediate climatic benefit of mCDR depends on air-sea CO₂ equilibration before the CO₂ depleted seawater deficit in the surface ocean loses contact with the atmosphere through water mass ventilation. Air-sea CO₂ equilibration occurs over vast ocean regions, which are too large to constrain equilibration with current observational methods. As such, numerical modeling is needed to evaluate the spatial and temporal scales of air-sea CO₂ equilibration. This study employs the ACCESS-OM2 model at three resolutions (0.1°, 0.25°, and 1°) to evaluate the dependency of simulated equilibration timescales on model resolution. Results indicate that model resolution has limited influence on equilibration timescales in the tropics but exerts a more significant effect in polar regions. The main reason for the simulated differences is that different resolutions advect CO₂-deficient seawater into different locations (horizontally and vertically) where air-sea exchange can occur at different rates. The comparison of our results with simulations made with other ocean models further suggests that differences due to model resolution are smaller than differences between different models of similar resolutions. Our results are one step forward in evaluating the robustness of model-based assessments of air-sea CO₂ equilibration timescales.

Surface phytoplankton biomass (measured in mol C m⁻³) represents a critical parameter within the Earth System that is measured from space and simulated in Earth System Models. Under climate change, the current generation of Earth System Models agrees that low-latitude biomass will decline and high-latitude biomass will increase. However, on a regional scale, the magnitude, phenology and spatial pattern of these changes are highly inconsistent across models. We use machine learning to investigate the sources of the divergence and evaluate the realism of the simulations. We train Random Forests driven by environmental drivers to simulate surface phytoplankton biomass under both pre-industrial control and SSP5-8.5 scenarios. Outside the Arctic, the bulk of the changes in biomass are driven by rearrangements in the spatiotemporal distribution of environmental predictors. Large regional changes in models, however, are associated either with unrealistically low pre-industrial levels of macronutrients or unrealistically strong responses to those macronutrients. Within the Arctic, relationships between environmental predictors and biomass change under global warming. While increased light drives increased biomass, the effect is largest in models with a high nutrient bias. Feeding inputs from an ensemble of models to an emulator trained on observations predicts observed biomass better than the ensemble of the models does, highlighting the fact that models do not produce the correct relationships between environmental predictors and biomass. However, this technique does not yield mechanistically consistent predictions of biomass under climate change. Skepticism of large regional changes in surface phytoplankton biomass produced by individual models is warranted.

Member countries of the Association of Southeast Asian Nations ratified the Paris Agreement and have initiated their own efforts to reduce greenhouse gas (GHG) emissions. However, the progress of these countries toward climate neutrality remains uncertain. Here, we estimated the combined budget for carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) in Southeast Asia for 2000–2019 using bottom-up and top-down approaches. The CO₂ emissions from deforestation were the largest source, followed by anthropogenic fire emissions, which together exceeded the CO₂ uptake by natural vegetation and land-use change legacy (e.g., regrowth), yielding a net source of CO₂ in the biosphere. The region's biosphere was also a net source of CH₄ and N₂O, which, combined with the CO₂ budget, makes the Southeast Asian biosphere a net source of GHGs to the atmosphere, ranging from 2,003.2 ± 406.1 Tg CO₂eq yr⁻¹ (bottom-up) to 2,227.5 ± 572.8 Tg CO₂eq yr⁻¹ (top-down) for 2000–2019. Among non-biospheric GHG emissions (e.g., fossil fuels and waste-related emissions), coal usage has resulted in an unprecedented increase in CO₂ emissions. The total GHG budget (the biospheric GHG budget plus the non-biospheric GHG fluxes) was calculated as a net source of 3,226.3 ± 406.2 Tg CO₂eq yr⁻¹ (bottom-up) and 3,406.4 ± 572.9 Tg CO₂eq yr⁻¹ (top-down) for 2000–2019. Our study revealed that Southeast Asia is experiencing the dual challenge of large emissions from deforestation and coal usage, necessitating the implementation of urgent mitigation strategies to ensure climate neutrality.

Soil organic matter (SOM) reserves in paddies are approximately two times larger than those in upland soils, and therefore, rice paddies have a strong impact on terrestrial carbon (C) sequestration. Functional partitioning of SOM into particulate organic matter (POM) and mineral-associated organic matter (MAOM) facilitates our understanding of C sequestration capacity in paddy soils. We analyzed POM and MAOM contents in 104 samples of topsoil and 81 samples of subsoil collected from paddies, and investigated how climate, nitrogen (N) fertilization, and soil depth regulate POM and MAOM storage. MAOM was the predominant fraction (45.3%–63.7%) of SOM in all paddy soils. As the SOC content increased, POM increased linearly, while the increase rate of MAOM slowed down, indicating a tendency for MAOM to reach saturation. The influence of mineral types on POM and MAOM protection exhibited depth-dependent patterns: clay minerals showed stronger associations in topsoil, whereas amorphous iron oxides displayed increasing importance in subsoil. Climatic factors, particularly mean annual temperature (MAT), had contrasting effects on POM and MAOM storage: increasing MAT reduced MAOM content and stability while having a minor impact on POM. Increasing the N application rate had minimal impact on POM and MAOM storage due to crop harvest and the balance between microbial activity and mineral protection mediated by soil acidification. These findings are valuable for facilitating the sequestration and increasing the stability of SOM in paddies, providing information for global soil carbon storage strategies.

Tides are a critical source of energy and influence transport in the ocean, with potential implications for biogeochemical cycling and biological production. Here, we quantify the influence of tides on nutrient transport and primary production in the northern Indian Ocean, where topographic features generate hotspots of tidal processes and primary production supports important regional food supply via aquaculture and fisheries. Using a high-resolution regional ocean model, we investigate the effects of diabatic tidal mixing (i.e., irreversible mixing that stirs nutrients) and adiabatic motions associated with internal tides (i.e., reversible motions that shift water masses without mixing) on primary production. We find that tides increase regional primary production by 5% on average, with seasonal increases reaching up to 10%–15% in open ocean regions and 30% in coastal regions. Tidal mixing sets the magnitude of tide-driven production by supplying nutrients to the euphotic zone, and controls the contrast between the stronger coastal response and milder open ocean response. Background stratification and nutricline depth control the seasonality in tide-driven production at a given location: tides accelerate the onset and delay termination of blooms in productive regions (e.g., Arabian Sea) and reinforce bloom peak in less productive regions (e.g., Bay of Bengal). Adiabatic motions have only a small effect, indicating that tidal influences can be effectively parameterized in global models without costly high spatio-temporal resolution and explicit tidal forcing. We discuss the influence of tidal mixing on biogeochemistry beyond primary production and how it may change in a warmer and more stratified ocean.

Continental shelf surface waters are considered a variable but increasing sink of atmospheric carbon dioxide (CO₂), but the mechanisms controlling these increasing sinks are unclear. We identify that the winter wind-driven surface atmosphere-ocean CO₂ gas exchange and wind-driven movement of water onto (or off of) shelf seas are consistent with the atmospheric CO₂ uptake tendency of many shelf seas. A 20-year observational-based analysis shows that geostrophic, wind and wave driven currents all contribute to the surface shelf break water velocities, but the dominance of each is location and season dependent. Analyzing these flows for fourteen shelf-seas based on their 20-year long-term gradient in air-sea partial pressure of carbon dioxide (their atmospheric CO₂ uptake tendency) identifies significant relationships between uptake tendency and winter (r² = 0.72 ± 0.03, p < 0.01, n = 14) and autumn (r² = 0.57 ± 0.05, p < 0.01, n = 14) wind-driven surface flows. These signals are most strong in winter, but the results are consistent at annual scales. Including the wintertime wind-driven air-sea CO₂ gas exchange further enhances this result, and collectively they describe 82% of the variance in the atmospheric CO₂ uptake tendency data (r² = 0.82 ± 0.06, p < 0.01, n = 14). These findings identify that long-term wind-driven water flow and surface gas exchange are key mechanisms for controlling their chemical evolution and their status as CO₂ sinks. This observational-based evidence highlights the need for these wind-driven processes to be resolved within methods used to predict or understand continental shelf-sea carbonate system state and ocean health.

Sustaining phytoplankton primary production and organic carbon export requires the physical supply of nutrients to the sunlit ocean. In the extensive downwelling regions of the subtropical gyres, the pathways of this nutrient supply remain unclear. Vertical sinking of organic matter from the sunlit layer and its remineralization below cause net downward nutrient transfer in the upper subtropical ocean. Microscale mixing of nutrients across density surfaces and upwelling by mesoscale eddies and submesoscale fronts have been invoked to re-supply nutrients from the thermocline to the sunlit layer. However, a physical mechanism is required to replenish nutrients exported across the thermocline base and sustain a quasi-steady state upper-ocean nutrient budget on inter-annual timescales. Stirring along density surfaces by mesoscale eddies has emerged as a possible supply mechanism to close this nutrient budget. Here, we quantify the relative importance of mesoscale stirring and microscale mixing in supplying nutrients to the oligotrophic regions of the upper subtropical oceans, using global observationally based data sets for nutrients and diapycnal and isopycnal diffusivities. Mesoscale stirring dominates nutrient replenishment in the thermocline of subtropical gyres over microscale turbulence, contributing to 70%–90% of combined supply by the two processes. The stirring supply is most important along gyre flanks, where boundary currents and upwelling zones promote sharp nutrient gradients and vigorous mesoscale activity. Mesoscale fluxes provide sufficient nutrients to offset depletion in the thermocline due to upward microscale mixing into the sunlit layer. This analysis suggests that eddy stirring is significant in maintaining organic carbon export within subtropical gyres.

Dissolved organic carbon (DOC) constitutes the largest pool of reduced carbon in the global ocean, with important contributions from both recently formed and aged, biologically refractory DOC (RDOC). The mechanisms regulating RDOC transformation and removal remain uncertain though hydrothermal vents have been identified as sources and sinks. This study examines RDOC sinks in the deep Pacific Ocean, highlighting the role of submarine hydrothermal systems. Geochemical survey data from GO-SHIP and GEOTRACES projects, alongside specific investigations of Pacific hydrothermal systems, suggest that particulate iron introduced by hydrothermal systems plays a key role in scavenging DOC and delivering it to the seafloor, leaving a deficit in the RDOC of the deep ocean. Dilution of the oceanic water column by hydrothermal fluids exhibiting low DOC concentrations likely plays a secondary role.